1. Field of the Invention
The present invention relates to a device for detecting thermal radiation, and to presence detection apparatus based on such a device. The invention can be used particularly, but not exclusively, for detecting intrusion into a security zone under surveillance.
2. Related Art
In the field of high-security surveillance, e.g. monitoring premises for storing nuclear material, it is essential for monitoring objects that are in the zone under surveillance failure and without interruption.
At present, in order to ensure continuity of knowledge concerning a situation that is under surveillance, e.g. a quantity of fissile material, and in order to keep a zone of finite dimensions under surveillance, use is made of systems based on seals, whether electronic or otherwise (with identity and/or integrity), of optical surveillance systems (using cameras), and of a range of conventional detectors.
Systems based on seals are generally insufficient because most of them can be used only once, and they are incapable of providing remote surveillance.
With video surveillance systems, the large amount of investment that is required, the image and signal processing that is necessary for detecting an intrusion, and the possibility of error by a spurious image, lead to enormous drawbacks when used in high security surveillance setups.
In most detection applications, traditional surveillance systems based on ultrasound are used for measuring radiation from moving sources. The principle on which they are based is difficult to use for remote surveillance while still allowing movement to take place in a portion of the same premises that is not under surveillance.
Surveillance by means of ultrasound detectors becomes impossible whenever some movement is to be expected around the boundaries of the zone under surveillance since such detectors respond specifically to movement and molecular vibration.
Numerous pyroelectrical and semiconductor devices are commonly used for detecting thermal radiation emitted by a moving source. Since thermal radiation varies with the emissivity of a surface, such detectors suffer from the major drawback of having their responsiveness dependant on wavelength. It is therefore necessary to select a detector as a function of the source to be detected since the detector operates in a narrow frequency band. In addition, detectors of those types are themselves heated by the radiation they receive, and they always require a cooling system.
It can thus be deduced that for an application seeking to provide high security surveillance of a specified zone, the systems mentioned above present at least one of the following drawbacks:
responsiveness is nearly always dependent on wavelength, thereby making it necessary to select a specific detector as a function of the source to be detected; PA1 with infrared or ultrasound systems, only movement is detected in the detection area. Such systems provide detection information due only to the movement of a source of thermal radiation in the zone under surveillance, but no information concerning presence; PA1 an optical lens is generally used for focusing incident rays, thereby giving rise to an undesirable filter effect; PA1 encapsulation is normally necessary in order to eliminate external noise resulting from convection currents; PA1 a cooling device needs to be provided in many cases; and PA1 with ultrasound systems, it is not possible to focus surveillance on a particular zone of finite dimensions without suffering disturbances from people or objects moving around the boundaries of said zone. PA1 a detector having one or more portions designed to be exposed to the thermal radiation, and one or more portions designed to be protected from the thermal radiation, the detector delivering a detection signal on the basis of a temperature difference between the exposed portions and the protected portions; and PA1 a reflector type concentrator associated with said detector to concentrate thereon the thermal radiation coming from a predetermined three-dimensional zone.
Thus, an object of the present invention is to provide a device for detecting thermal radiation that does not suffer from the above-mentioned drawbacks.